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Advertisers have long known about a problem with mobile ads: fat fingers.

That is, people accidentally click an ad on those little smartphone screens thanks to clumsy digits (or purposely big or hard-to-avoid ads). Realizing their mistake, they back up instantly, but the advertiser gets charged while getting only a wisp of attention from a consumer they probably didn’t want to reach anyway. Today, Google is introducing a tweak to in-app image ads that should reduce those unintentional clicks considerably.

It’s a big issue. Recent studies indicate that up to about 40% of mobile ad clicks are accidental or even fraudulent, based on the fact that people “view” the ad two seconds or less. The result, of course, is not only that advertisers get charged even though consumers had no interest in the ad, but that they obviously aren’t going to end up buying the product or service.

This may be one reason advertisers pay much less per impression for mobile ads. And that’s a problem that has investors concerned about every company from Google and Facebook to a raft of mobile and app startups, as more and more online activity moves from stationary computers to smartphones and tablets.

Google found most of the accidental clicks on app image ads happened at the outer edges of the ad, no doubt because people were trying to scroll up or click on adjacent content. So now, Google has added a prompt to “Visit site” whenever people click on the outskirts of the ad. It’s an extra click, but it also ensures that’s really what the person wanted to do. …